At the time the guidelines were written at the beginning of 2013, they noted that the "pirate business model" in the Gulf of Guinea "does not primarily involve kidnap for ransom, therefore the crew of a ship does not in itself represent the 'value.'"

But the guidelines said that pirates in the Gulf of Guinea were more violent than the Somalia-based pirates on the other side of the African continent -- a much more publicized hotbed of piracy -- and instructed crewmembers "not to engage in a fight with the pirates, because this will entail great risk of the crew getting hurt or killed."

"Great care needs to be taken if your ship is boarded, as life is little valued by robbers," the guidelines say. "Compliance/submission to attackers is essential once a vessel has been taken."

Jack Cloonan, a former senior FBI agent now with Clayton Consultants, which specializes in international piracy, kidnapping and extortion, told ABC News Thursday the fact that the attackers reportedly singled out the Americans showed that the kidnappers do attach some value, at least monetary, to those lives.

"If you take the Americans, you get a good price, but at the same time you bring a lot of heat on you too," Cloonan said . "The initial demands will probably be ridiculously high and you can infer from that who you are dealing with. Are these people skilled? Do they have professional negotiators?... Do we know who this group is and is their end game actually money? Because if it is, I'm happy. I'm pleased and now I know it can be a negotiable end."

Neither the victims nor the attackers have been publicly identified. The company that owns the targeted ship, Edison Chouest Offshore, did not respond to comment Thursday. Representatives for both the White House and the State Department said Thursday their respective departments were "closely monitoring" the situation.